1. Field of the Invention
The present invention relates to an ion source device, and more particularly, to an ion source device suitable for an ion implantation apparatus and an ion beam generating method.
2. Description of Related Art
An ion source device for an ion implantation apparatus which includes an electron source and a repeller for reflecting electrons from the electron source is known (Japanese Unexamined Patent Application Publication No. 2002-117780).
Hereinafter, an example of an ion source device will be described with reference to FIGS. 3A and 3B.
In FIGS. 3A and 3B, the ion source device includes an arc chamber 20 having a space for plasma formation. The arc chamber 20 includes a front slit 20-1 in a wall of a front face thereof and an introduction unit 20-2 of a source gas in a wall of a lateral face thereof. In addition, in the ion source device, an electron source is installed at one of opposed locations of the arc chamber 20 with the space for plasma formation that is interposed therebetween, and a repeller 23 is installed at the other one thereof. The electron source includes a filament 21 and a cathode 22. As shown in FIG. 3B, in front of the front slit 20-1, a suppression electrode 24 and a ground (GND) electrode 25 which each have an opening through which an ion beam passes are disposed in parallel with each other in an extraction direction of the ion beam.
The ion source device is operated as follows. First, the filament 21 generates heat by a filament power source 26 so as to generate thermal electrons in a tip of the filament 21. The generated thermal electrons are accelerated by a cathode power source 27 so as to collide against the cathode 22, and the cathode 22 is heated by heat generated at the time of collision. The heated cathode 22 generates thermal electrons. The generated thermal electrons are accelerated by an arc voltage of an arc power source 28 that is applied between the cathode 22 and the arc chamber 20, and the generated thermal electrons are then discharged into the arc chamber 20 as beam electrons having energy sufficient to ionize gas molecules.
Meanwhile, in the arc chamber 20, a source gas is introduced from an introduction unit 20-2, and an external magnetic field F is applied. In addition, in the arc chamber 20, the repeller 23 is provided so as to face a thermal electron discharge face of the cathode 22. The repeller 23 has a function to reflect electrons. A direction of the external magnetic field F is parallel to an axis that connects the cathode 22 and the repeller 23. For this reason, the beam electrons that are discharged from the cathode 22 reciprocate between the cathode 22 and the repeller 23 along the external magnetic field F, and then collide against source gas molecules that are introduced into the arc chamber 20 so as to generate ions. As a result, plasma is generated in the arc chamber 20.
Since the beam electrons are present within a nearly-limited range by an applied magnetic field, ions are mainly generated within the range, and the ions arrive at an inner wall of the arc chamber 20, the front slit 20-1, the cathode 22, and the repeller 23 by diffusion and are lost in a wall surface.
On the other hand, an ion beam is extracted by passing through a slit that is parallel to a magnetic field from plasma that is diffused to the front slit 20-1. A current of the ion beam that is extracted (extraction current) depends greatly on plasma density in the front slit 20-1. For example, if the plasma density in the front slit 20-1 is high, an extractable beam current (extraction current) increases.
Incidentally, it is not likely that plasma diffusion in a direction at a right angle to the external magnetic field F occurs in the arc chamber 20, compared to plasma diffusion that is parallel to the external magnetic field F. Accordingly, plasma density rapidly decreases in the direction at a right angle to the external magnetic field F. In an ion source device of the related art, a beam extraction portion is placed at a location where plasma diffuses in the direction at a right angle to the external magnetic field F. In other words, the front slit 20-1 is provided in a wall of the arc chamber 20 in a direction perpendicular to a direction of the external magnetic field F. For this reason, the plasma density in the front slit 20-1 decreases, and thus the amount of ion beam that is extracted, that is, the extraction current, is also limited.
In the present situation, in order to increase a drawing beam, that is, in order to increase the plasma density in the front slit 20-1, a method of, for example, increasing a thermal electron current from the cathode 22 is adopted. As a matter of course, this causes a reduction in the life-span of the cathode 22 because the plasma density in the cathode 22 and the repeller 23 also increases.
In an ion source device such as an ion implantation apparatus, from the point of view of an increase in productivity, it is required to further increase an extraction current from the ion source device. In order to increase the extraction current, it is required to generate plasma with a higher density in the vicinity of an ion beam extraction portion (front slit) of the ion source device. Accordingly, it is required to supply high power to the ion source device.